Today's digital radiotelephone systems are designed for full-rate coded speech while envisioning half-rate coded speech as advances are made in speech coding technology. Three such systems are the GSM, Pan-European Digital Cellular system, the US Digital Cellular system (IS-54) and the Japan Digital Cellular system, all Time Division Multiple Access (TDMA) radiotelephone systems.
The GSM system, for example, was designed around a full-rate speech coding technique where each (20 ms) block of (continuous) speech is digitized and error encoded for radio transmission in a repeating TDM time slot over eight frames--a so-called interleave depth of eight. In other words, it takes eight frames to recover all segment of the original 20 ms block of speech. The transmission bit rate and frame length is such that the delay between the speech being spoken and being received and reconstructed (actually, the complete round-trip delay) is virtually imperceptible. If the interleave depth were much greater, the delay would become annoying and the conversation would not be perceived to be taking place in "real time." If the interleave depth were much shorter, the delay would become inconsequential, but more than 12.5% (one-eighth) would be lost every time that one frame of information was corrupted. This would be perceived as poor audio quality.
To increase the capacity of these TDMA systems, technologists continue to pursue ways to encode speech at lower and lower bit rates. If the speech could be encoded into half the number of bits, twice as many users could be accommodated in the same radio spectrum. These systems traditionally envision such half-rate encoded speech and anticipate the availability of this half-rate speech coding in their system designs. Typically, as in the GSM system, it is envisioned that while the full-rate user will use every frame for speech transmission, the half-rate user will use every other, alternate frame for its transmissions. This permits another half-rate user to use the intervening, alternate frames, thereby doubling the system capacity. Unfortunately, the use of every other frame has the interrelated effects of either lengthening the delay or reducing the interleaving depth (where the loss of a single frame results in the loss of a proportionately higher mount of intelligible speech).
This invention takes as its object to overcome these shortcomings and to realize certain advantages presented below.